1. Field of the Invention
The present invention relates to a vehicle and a method for operating an engine in a vehicle.
2. Background Art
An engine in a vehicle can be operated in various ways to help minimize emissions. For example, operating the engine such that combustion is stable will help reduce emissions. To maintain stable combustion, a minimum amount of air flow into the engine must be maintained. This engine operating point may be referred to as a “combustion stability limit.” The combustion stability limit may be defined in terms of air flow, or the air flow can be translated into an engine brake torque value. Thus, a minimum engine torque may be set to ensure stable combustion. This torque value, called the “misfire torque limit,” is the lower bound of the engine torque production. In many vehicles, a buffer is provided such that the misfire torque limit is set above the true combustion stability limit. This buffer helps to ensure that the vehicle will not operate in the unstable combustion region, despite age and wear on the engine components.
In a vehicle such as a hybrid electric vehicle (HEV), very low engine torque outputs may be required under certain conditions. In an HEV, excess torque produced by the engine may translate into a charging current into the battery. If the battery charging is unchecked, the battery may overcharge, leading to reduced battery performance and battery life. Because of these battery constraints, it is desirable to set the misfire torque limit as low as possible—i.e., the buffer between the misfire torque limit and the true combustion stability limit should be reduced or eliminated. Because the misfire torque limit may increase as the engine ages, a method for adjusting the misfire torque limit is needed to satisfy the dual goals of keeping the misfire torque limit low, while not allowing the engine to operate below the true combustion stability limit.
One system that can be used to deal with engine misfires is discussed in U.S. Patent Application Publication No. 2003/0173123, filed by Nakanowatari and published on Sep. 18, 2003. The system described in Nakanowatari includes an engine misfire determination section that determines whether an engine misfire has occurred. The control system also includes a control section that permits operation of only the motor to propel the vehicle, while inhibiting operation of the engine, when it is determined that a misfire has occurred. If it is determined that the battery is in need of charging, the engine may be allowed to operate in order to charge the battery before the motor takes over. One limitation of the system described in the Nakanowatari is that it does not provide a means to help ensure that the engine is operated only in those states where misfires are unlikely to occur.